The present invention relates to the area of evaluating water quality for subsurface injection.
The present invention relates to the field of determining water quality for subsurface injection using membrane filters, following the title of the NACE standard Methods for Determining Water Quality for Subsurface Injection Using Membrane Filters, National Association of Corrosion Engineers, NACE Standard TM0173-84 (1976 Revision), hereinafter referred to as TM0173-84. This test is relevant in oil production fields where fluids produced from the wells typically contain large portions of water (for example, water-to-oil ratios of 10:1 are not uncommon). Disposal of the water has always been a problem, and standard practice has been to reinject the water back into the ground via a separate injection well. This practice has the added benefit of acting as a drive fluid to push more oil to the producing well. However, it is important to know some properties of the injected water since the injection practice may in fact damage the well. Standard TM0173-84 has been developed to provide information regarding some of these properties to operators in the field.
Specifically, TM0173-84 outlines two (2) tests. Quoting from TM0173-84, Section 3, page 1:
3.1 Rate v. Cumulative Volume Test PA0 3.2 Suspended Solids Test PA0 3.3 The suspended solids test and the rate volume test can be run concurrently. PA0 6.3.1 The test pressure for rate vs. cumulative volume should be 20 psig (138 kPa).+-.10% at the membrane. Because permeability variations can occur with pressure-sensitive filter cakes, it is advisable to determine if a test conducted at higher constant pressure is more definitive of the effective water quality in the system. (Emphasis added). PA0 6.3.2 Test pressure is obtained by suitable mechanical devices for on-stream sampling and by a pressure regulator between the nitrogen source and the sample reservoir for pressurized tests. PA0 6.4 Sample Volume
3.1.1 This test method consists of passing a fixed volume of injection water through a membrane filter under constant pressure and measuring the flow rate and cumulative volume of water at intervals. PA1 3.1.2 This test is primarily designed for monitoring injection water quality. A plot of the flow rate vs. cumulative volume of water gives a general indication of the injected water. PA1 3.2.1 This test method consists of collecting samples of primary solids (as defined in Section 2.1.1 of TM0173-84) as they exist in a water system. The suspended solids from several liters of water are collected on a membrane filter in a manner that permits larger, more representative samples than those obtained from bottle samples. PA1 6.4.1 For routine rate vs. cumulative volume tests, the sample size shall be 2.5 L (liters). Where water quality permits rapid filtration, samples up to 10 L or more may be required for meaningful interpretation. (Emphasis added). PA1 1. The need for an infinite volume sample apparatus that will allow simple, reliable constant pressure control; PA1 2. The need for a constant pressure sample apparatus that will allow infinite volume samples;
Section 6 of TM0173-84, "Test Conditions," states:
FIG. 2 of TM0173-84 discloses an apparatus for providing a sample of infinite volume, if required, as suggested in Section 6.4.1, above. However, the only mechanism for controlling pressure on the membrane filter as required in Section 6.3.1 is the manual valve at the sample source. Manual control of pressure requires attention each time the test is run to assure the initial pressure is correct and to monitor the pressure to guarantee that unacceptable variations (.+-.10%, per Section 6.3.1, above) do not occur. These pressure variations are possible if the pressure in the sample manifold in the apparatus in TM0173-84 FIG. 1 varies. Manual control may not be quick enough to adjust for pressure variation, and depending on the type of valve used at the sample point, precision of control may be unacceptable. Summarizing, manual control of the pressure over the membrane can be both labor intensive and inaccurate. A better means of controlling the required constant pressure in an infinite volume sample apparatus is needed.
FIG. 3 of TM0173-84 discloses an apparatus for maintaining constant pressure over the membrane by applying a pressure source such as compressed nitrogen gas. This apparatus has some inherent drawbacks. First, if a higher pressure is desired, as suggested it might in TM0173-84 Section 6.3.1, some means of controlling the pressure over the membrane is needed. While no means is disclosed in the FIGURE shown, it is conceded that means known to the art could be employed, such as putting an adjustable pressure controller on the nitrogen supply line, or putting an adjustable pressure relief device on the calibrated reservoir. A further constraint of this apparatus is that it is volume limited. This creates two problems. First, if a larger sample is desired, as suggested by Section 6.4.1, a larger reservoir may be needed. Thus, the apparatus must be constructed with the maximum volume anticipated in mind, or, retrofits at time and expense may be required. Second, if during a test it is determined that a larger volume is in fact needed, the test must be aborted and run again with the larger volume. This will result in a time intensive trial-and-error method to optimize the volume desired. The volume limited sample reservoir also introduces the chance for the reservoir to empty and nitrogen gas to blow directly through the membrane. This may damage the membrane and would at least make any test results highly suspect. Consequently, constant attention to the apparatus is required. Summarizing, again the apparatus disclosed in FIG. 3 of TM0173-84 is both labor intensive and subject to error. A better method of accommodating sample volumes in a constant pressure test apparatus is needed.
Sometimes a modified sample of the injection water is desired. Standard practice in the oil field is to extract oil (and gas) from the fluids extracted from the formation and then inject the produced water back into the formation to act as a drive mechanism to enhance oil recover. This injected water is the water that is the subject of the test in TM0173-84. However, if the water is sampled before the oil is removed, it is not a representative sample of the injected water. Sometimes it is not practical to sample the water after oil removal. If this is the case, then an "oily" sample must be drawn, decanted, the oil removed, and then tested. But produced water contains many chemicals which may react with oxygen (for example, free iron) but don't because the production-extraction-injection process is a closed process. Therefore, the decanted sample must be kept in an oxygen free atmosphere. This is technically not difficult, but is a great inconvenience for the operator running the test, particularly in the oil field. Thus what is needed is an apparatus that will allow on line sampling of produced water before oil is extracted.